1. Field of the Invention
This invention relates to radio frequency communication systems including transceivers and, more particularly, to the generation of calibration tones for receiver image rejection calibration.
2. Description of the Related Art
In many modern communication systems, information is transmitted and received by modulating a radio frequency (RF) carrier signal with a data signal and then demodulating the RF signal to recover the data signal. Demodulating an RF carrier signal usually involves tuning a receiver to the carrier signal, which may be at a substantially higher frequency than the frequencies of the data it conveys. For example, a carrier signal frequency may be on the order of 1-2 GHz, whereas the bandwidth of the data channel conveyed by the carrier-signal may be on the order of only 100-200 kHz.
In one approach to demodulating an RF signal, a bandpass filter having a bandwidth approximately equal to the data channel bandwidth may be tuned to the RF carrier signal frequency, and the filter output may be demodulated according to the original modulation scheme. However, narrow-bandwidth bandpass filters become increasingly difficult and bulky to implement as their center frequencies increase. Consequently, many RF receiver systems use some form of heterodyning to convert a received RF signal to a lower frequency (referred to as an intermediate frequency, or IF), where the task of filtering may be easier.
Generally speaking, heterodyning a signal involves mathematically multiplying one signal, such as an RF signal, with a second signal close in frequency to the first, often referred to as a local oscillator (LO). Such an operation, also referred to as mixing, results in signals at frequencies equal to the sum and difference of the RF frequency and the LO frequency. The sum frequency is usually substantially higher than the RF or LO frequency and may be readily filtered using a simple low-pass filter. The difference frequency is the IF frequency, which may be close to DC and is therefore readily manipulated by simple filters.
Typical heterodyne systems are susceptible to a phenomenon referred to as imaging. As described above, a given desired RF frequency fRF differs from a given LO frequency fLO by the IF frequency fIF. Further, a given desired RF frequency may lie either above or below the LO frequency. However, due to its symmetric properties, heterodyning will select any RF signal differing from fLO by fIF, irrespective of whether the RF signal lies above or below the given LO frequency. In heterodyne systems generally, for a given RF signal of frequency fRF=fLO±fIF, the frequency fimage=fLO∓fIF may be referred to as the image frequency.
To prevent interference with the desired RF signal, the image frequency may be filtered prior to heterodyne mixing. In some systems, quadrature receiver architectures may be employed that facilitate image frequency rejection by splitting the desired RF signal into two paths and mixing each path with a respective function of the LO signal, where the respective functions may have a particular phase relationship (such as sine and cosine functions). One of the paths is typically referred to as the in-phase (I) signal path while the other path is referred to as the quadrature (Q) signal path. Quadrature IF mixing inherently provides a property that makes the cancellation of the image signal possible without the use of expensive and bulky image filters. For example, when one of the I and Q signals output from the IQ mixer is phase shifted and the I and Q signals are summed, the image frequency may be cancelled. However, imbalances in the gain and phase relationships between the two paths may yield imperfect image frequency cancellation, and thus result in a residual image frequency. This residual image frequency may cause undesired interference that may limit the performance of the receiver.
Accordingly, to accommodate improved rejection or cancellation of the image frequency, some receivers may use a calibration tone to calibrate the receiver to account for imbalances in the gain and phase relationships between the I and Q paths. For example, in some systems, a calibration tone generated at the image frequency may be provided to the input of a quadrature mixer during a calibration mode. A residual image signal may then be measured to derive appropriate gain and/or phase adjustments to be applied to the I and/or Q signal paths to thereby attain improved image rejection capabilities.